Priority to German Patent Application 102005037717.3-52 is claimed, the disclosure of which is incorporated herein by reference.
The present invention relates to a method for diagnosing an external temperature sensor of a motor vehicle and a device for carrying out this method.
U.S. Pat. No. 6,684,154 B2 discloses checking the satisfactory functioning of an engine temperature sensor by storing the last temperature value when the engine is switched off. When the engine is switched on later, a difference between the stored last temperature value and a measured current temperature value is compared with a difference which is formed from the last stored temperature value and a calculated temperature value, the calculated temperature value being calculated as a function of operating conditions of the engine and as a function of the time period between the switching off and the switching on of the engine.
U.S. Pat. No. 5,995,887 discloses checking the functioning of a transmission temperature sensor by determining a transmission oil temperature with the transmission temperature sensor when the vehicle starts and comparing it with a predefined lower limiting value as well as with a predefined upper limiting value. If the measured temperature lies below the lower limiting value or above the upper limiting value, a malfunction is detected if the temperature measured with the transmission temperature sensor does not change by a predetermined value during the further operation of the vehicle.
German Patent Document DE 38 30 389 C2 discloses checking the functional capability of a temperature sensor in a temperature-controlled heating device by virtue of the fact that a control unit has a heating element energy counter and a counter controller for incrementing and decrementing the heating energy counter at speeds which are a function of the power absorbed by the heating element, the counter reading of the heating element energy counter following the temperature of the heating element. A malfunction of the temperature sensor occurs if the counter reading of the heating element energy counter lies within a predetermined reference range and the measured temperature lies outside a correlative predetermined reference temperature range.
German Patent Document DE 103 29 038 B3 tests an engine temperature sensor for a defect on the basis of the change in the sensed temperature. The sensor is defective if the measured temperature does not change by a certain minimum absolute value within a predefined time. It is assumed that a motor sensor which does not react is defective. However, this method is unsuitable for external temperature sensors since the measured external temperature can easily remain the same over several hours.
German Patent Document DE 199 51 788 C2 compares the measured values of two independent temperature sensors with one another in order to decide whether a sensor is defective. However, an increased amount of expenditure on hardware is necessary in this context.
Japanese Patent Document JP 2004325110 A discloses a temperature sensor for batteries, a plurality of which can be connected in series. A fault is detected if the change of temperature exceeds a value. However, there is no differentiation between two different faults or groups of faults on the basis of the sign of the temperature gradient. The method cannot be transferred to an external temperature sensor.
German Patent Document DE 101 12 139 A1 as well as DE 103 16 606 A1 disclose further methods and devices in which, in order to check a temperature sensor, the measured temperature values of at least two different temperature sensors are compared with one another.
External temperature sensors in motor vehicles must operate reliably over relatively large temperature ranges. For example, temperature sensors have to supply reliable temperature values between −40° and +85° C. A malfunction of the external temperature sensor can be detected, for example, if the measured external temperature lies outside the aforesaid external temperature measuring range. For example, temperatures of higher than +85° C. and lower than −40° can be triggers for the setting of a fault signal. In modern motor vehicles, such a fault signal is stored centrally and appears automatically when the vehicle is next serviced. As result, the external temperature sensor is replaced, which is associated with corresponding expenditure. However, it has become apparent that temperatures below −40° C. can actually occur at least in some geographic zones so that temperatures outside the temperature measuring range cannot necessarily be attributed to a malfunction of the external temperature sensor. For this reason, a possible way of being able to detect malfunctions of the external temperature sensor more reliably is sought.
The present invention is concerned with the problem of specifying an improved method and device for diagnosing an external temperature sensor of a motor vehicle, which can detect, in particular, the presence of a malfunction with increased reliability and can preferably differentiate the occurrence of extreme temperatures from a fault.
The invention is based on the concept of comparing a temperature gradient determined with the external temperature sensor with a limiting gradient in order to assess the reliability of the measured temperatures. If the measured temperature rises more quickly or drops more quickly than the limiting gradient, a malfunction has occurred. The invention makes use here of the realization that the external temperature sensors which are generally used can only follow extreme temperature differences at a limited maximum speed owing to their physical properties. The aforesaid limiting gradient is aimed at this maximum realizable temperature change per time unit. If the measured temperature change per time unit is greater in absolute value than the maximum temperature change per time unit which can be realized physically by the external temperature sensor, a malfunction of the external temperature sensor or of the entire arrangement for measuring the external temperature has inevitably occurred.
In the case of an external temperature sensor which operates with a Negative Temperature Coefficient (NTC) element, the temperature is determined as a function of the electrical resistance of the NTC element. As the temperature drops, the resistance of the NTC element rises. A cable break within the measuring arrangement gives rise to an extremely steep rise in the resistance and accordingly to an extremely steeply dropping temperature gradient which is at any rate greater in absolute value than the limiting gradient. The NTC element is a thermistor which is composed of a semiconductor resistor which senses temperatures.
According to one embodiment, a first fault is detected if the determined temperature gradient is positive and greater in absolute value than the limiting gradient, while a second fault is detected if the determined temperature gradient is negative and greater in absolute value than the limiting gradient, is of particular advantage. In this embodiment, faults which arise when there are positive temperature gradients can thus be differentiated from faults which arise when there are negative temperature gradients. As a result, it is possible to refine the diagnosis, which simplifies the possibility of recovering from faults.
Of course, the features mentioned above and to be explained below can be used not only in the respectively specified combination but also in other combinations or alone without departing from the scope of the present invention.
Preferred exemplary embodiments of the invention are illustrated in the drawings and will be explained in more detail in the following description in which identical reference symbols relate to identical or similar or functionally identical components.